Joeish W <joeish80...@yahoo.com> writes:
> I have this C wrapper
>
>
> DMatch* cv_create_DMatch3(int _queryIdx, int _trainIdx, float _distance) {
> return new DMatch(_queryIdx, _trainIdx, _distance);
> }
>
>
> it is a wrapper for these struct members
>
> http://docs.opencv.org/modules/features2d/doc/common_interfaces_of_descriptor_matchers.html?highlight=dmatch#DMatch
>
>
> here is my lisp wrapper for it:
>
> (defctype dmatch :pointer)
>
> (defcfun ("cv_create_DMatch3" dmatch3) (:pointer dmatch)
> "DMatch constructor"
> (-query-idx :int)
> (-train-idx :int)
> (-distance :float))
>
> In C I can do this:
>
> DMatch* a = cv_create_DMatch3(1, 2, 3.0);
> cout << a->distance ;
>
> and the output is 3
>
> but in lisp when I do this:
>
> (defparameter a (dmatch3 1 2 3f0))
>
> LCV> (mem-aref a :int 0)
> 1
> LCV> (mem-aref a :int 1)
> 2
> LCV> (mem-aref a :float 2)
> #<SINGLE-FLOAT quiet NaN> <--- this is output
>
>
> Any help on figuring out the reason would be much appreciated
This is not really suprising. You are treating the resulting pointer
`a` as an untyped pointer.
When you do `(mem-aref a :float 2)` you are basically saying:
1. Assume a is a pointer to an array of floats
2. Give me the third element (element at index 2).
Unfortunately `a` is not a pointer to an array of floats, but a pointer
to a struct DMatch.
You should lookup `defcstruct` in the CFFI documentation:
http://common-lisp.net/project/cffi/manual/html_node/defcstruct.html#defcstruct
Wim Oudshoorn.
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